Suspected Taliban gunmen torched 50 Nato supply trucks on the outskirts of Islamabad last night , killing at least seven people, in the most audacious assault yet on the military supply chain in Afghanistan.

Militants have regularly ambushed western supply convoys near the Afghan border, but this was the first attack near the Pakistani capital.

Police said at least 10 gunmen struck just before midnight at a depot in Tarnol, on the Grand Trunk Road that links Rawalpindi with Peshawar. They opened fire on the trucks, killing drivers and security personnel, before setting them alight.

Police arrived, sparking an exchange of fire, and the attackers escaped to the west on motorbikes and in small pickup trucks. At first light, six hours later, flames were still licking through the hulks of charred military vehicles and fuel tankers. Plumes of thick black smoke rose into the sky near the capital.

The assault called into question security arrangements in the capital, whose streets have become clogged with police and military checkposts since the Taliban stepped up their assault last year.

Police officers combing the area said they had arrested 26 suspects. "We are still trying to find out what happened," said Shah Nawaz, the police chief at Tarnol station.

Pakistani police often indiscriminately arrest people after major incidents only to release them later. The interior minister, Rehman Malik, ordered an inquiry within three days.

The convoy originated in the port of Karachi, 700 miles to the south, and was due to climb over the Khyber Pass into Afghanistan. Over 75% of military supplies for western troops in Afghanistan and 40% of fuel needs pass through Pakistan.

The supplies include food and vehicle but not usually weapons or ammunition, which are ferried by air.

The last attack was in April when militants torched a dozen trucks and killed four police in northern Punjab, between Islamabad and Peshawar.

Many of the Nato supply runs are managed by trucking families from the tribal belt, particularly Waziristan and Khyber. A police source in Karachi said they ensured safe passage by paying protection money to militant groups on the 1,200-mile route.

However, it is impossible to pay off every group along the road. "When you reach here, it's a free for all," said Rahimullah Yusufzai, a veteran journalist in Peshawar. "You can't pay everyone off; and some groups are ideologically committed and won't take money."

The attacks are a costly irritant to Nato and an embarrassment to Pakistani security forces, although they have a limited impact on western military supplies due to the high volume of traffic.

Still, the US has started to explore new trucking routes through Central Asia, although these may require compromise with Russia, the regional power.